In my case I'd like to be able to add `-m last` to `cage` to make sure
that the login form from regreet isn't displayed half on my external
monitor and half on my laptop screen, but on the last connected monitor
only.
That's basically the issue described in #226586, though it's not a
proper fix since the login form is shown on one monitor only.
Prior to this commit the derivation assumed a user's primary group has
the same name as the user themselves. This is standard on linux but not
necessary (and indeed I believe not the default on NixOS).
Closes#232184
